The U.S. military’s workhorse rifle — used in battle for the last 40 years — is proving less effective in Afghanistan against the Taliban’s more primitive but longer range weapons.

As a result, the U.S. is reevaluating the performance of its standard M-4 rifle and considering a switch to weapons that fire a larger round largely discarded in the 1960s.

The M-4 is an updated version of the M-16, which was designed for close quarters combat in Vietnam. It worked well in Iraq, where much of the fighting was in cities such as Baghdad, Ramadi and Fallujah.

But a U.S. Army study found that the 5.56 mm bullets fired from M-4s don&apos;t retain enough velocity at distances greater than 1,000 feet (300 meters) to kill an adversary. In hilly regions of Afghanistan, NATO and insurgent forces are often 2,000 to 2,500 feet (600-800 meters) apart.